1. Field of the Invention
This invention relates to a method for preparing inorganic oxide-based materials of typically spherical form with controllable particle size, preferably monodispersed within a narrow diameter distribution band, the method consisting basically of forming a uniform finely divided dispersion of a sol of at least one of the oxides of interest in a solvent immiscible with the sol and chosen to allow limited coalescence in an emulsion in it, effecting growth by limited coalescence of the dispersed phase to a predetermined diameter by controlling for this purpose the time of growth of said dispersed phase, then causing gelling of the mixture thus obtained by adding a second solvent, and drying the resultant gel.
The method in question enables inorganic materials, typically oxides or oxide mixtures, to be rapidly and advantageously obtained, these forming a second aspect of the present invention and finding application in all those sectors in which high chemical and thermal stability, monodispersed particle size, spherical form and high porosity and surface area are essential requirements.
Of all possible uses, their application to the catalysis field is of particular importance, in which the materials of the present invention can be used as high-quality catalyst supports, especially in olefin polymerization processes using Ziegler-Natta or Phillips catalysts or in such processes conducted in the gaseous phase.
The materials of the invention can also be conveniently used in the insulation sector, in which a material of very low density, low cost and easily processable can certainly compete with foamed polyurethane. In this respect, a spherical monodispersed granular material is runnable and can be treated in many processes as a liquid.
Lastly, but of no less importance, the materials of the present invention can be applied in the ceramics field, in which precursor quality and particle size control are essential.
2. Background of the Invention
The use of inorganic oxides, especially metal oxides, is well known in the state of the art in relation to a large number of applications. For example, they can be used as catalysts, catalyst supports, adsorption media, packing for chromatographic columns, pigments, opacifiers, abrasives etc.
Examples of their use as catalysts or catalyst supports are given in U.S. Pat. No. 3,673,111 (phosphoric acid preparation) and in an article in the Journal of Molecular Catalysis, vol. 17, 219-223 (1983) (catalyst support in olefin preparation). Known powders, especially those for use as catalyst supports, normally consist of inorganic oxides obtained by precipitating hydroxides from aqueous solutions of the corresponding cations, followed by calcining. Powders obtained in this manner generally have a very low specific surface and porosity, the spherical particles do not always posses suitable mechanical properties, and their particle size is dispersed within a wide range, which sometimes makes their use inadvisable precisely for the application for which they are intended. In this respect, it is not infrequent to find that catalyst systems obtained using such powders do not allow the reactions of interest to take place with good productivity, so making it necessary to use increasing catalyst concentrations, requiring the application of costly wash processes for the catalyst residues.
It is likewise known to prepare aerogels and in particular aerogel microbeads of one or more oxides, the microbeads being characterised by high porosity and the fact that most of the pores have a diameter within a very narrow band, and being usable as catalysts or catalyst supports U.S. patent application Ser. No. 07/690,305. The preparation process for such microbeads requires however a drying stage under hypercritical conditions, which makes it not always suitable for convenient industrial application.